A Digital Signal 3 (DS3), also called a T3, is a legacy digital telecom circuit with a line rate of 44.736 megabits per second, enough to carry 28 T1 lines or 672 voice channels. For years it was the step up for a business or carrier that had outgrown a bundle of T1s, used for internet access, private lines between sites and large phone trunk groups. DS3s are now legacy services that many carriers are retiring or repricing, and most buyers meet them as a circuit to replace rather than one to buy.
At a glance
- A DS3 runs at about 45 Mbps and carries 28 T1-rate channels (672 channels of 64 kbps).
- DS3 is the signal; T3 is the common name for the circuit. People generally use the two interchangeably.
- It is usually fed over fiber and handed off on coaxial cable, or carried as a channel inside a larger optical circuit.
- Many carriers are retiring or raising prices on legacy circuits like DS3s, so they often appear on replacement lists.
- Ethernet services such as dedicated internet access or an Ethernet private line are the usual replacements for data.
What problem it solves
Before Ethernet services were widely available outside the local network, a business that needed more than a few megabits between sites, or to the internet, had limited options. Bonding T1s together worked up to a point, but each T1 was a separate circuit to order, bill and repair. A DS3 combined the capacity of 28 T1s on one circuit, giving larger offices, call centers, hospitals and carriers a single, symmetrical connection with a service level agreement.
Today the problem has reversed. At around 45 Mbps, a DS3 is slow by current standards, and the time-division multiplexing (TDM) equipment it relies on is costly for carriers to maintain. Ethernet over fiber often delivers many times the bandwidth for a similar or lower monthly charge. For buyers, a DS3 is usually a cost and risk to plan out of the network.
How it works
The digital hierarchy. In the North American system, a DS0 is one 64 kbps channel, a DS1 (carried on a T1) combines 24 DS0s, and a DS3 combines 28 DS1s, with extra bits for framing and alignment. That is why the line rate (44.736 Mbps) is higher than 28 times 1.544 Mbps.
Channelized or clear-channel. A channelized DS3 is split back into its T1s or voice channels at each end, which suits phone trunks or connecting several T1-based sites. A clear-channel DS3 delivers the whole rate as one pipe for data, such as internet access or a private line. A fractional DS3 sells only part of the capacity.
Delivery. A DS3 is normally handed off on a pair of coaxial cables from carrier equipment in the building, with a short distance limit between that equipment and your router or switch. Behind that handoff, it usually travels over fiber, often as one channel inside a SONET circuit. Your router needs a DS3 interface, which many current routers no longer offer as standard.
Outside North America. The comparable circuit in much of the rest of the world is the E3, at 34.368 Mbps. The two are not interchangeable.
If you are reviewing legacy circuits at a site, our Internet Access solution page covers the options that typically replace them.
When it matters for buyers
- Carrier retirement notices. In the US, carriers have been retiring legacy TDM and copper services or raising their prices. Timing varies by carrier and location, so check notices and contracts and plan replacements before a forced deadline.
- Inherited sites. After an acquisition or a staff change, DS3s can stay on the bill long after their purpose is forgotten. Match each circuit to what is actually connected.
- Bandwidth upgrades. A site still on a DS3 for internet usually gains a lot of capacity by moving to dedicated internet access or business fiber.
- Site-to-site links. A DS3 private line between offices or a data center is often replaced by an Ethernet private line or another private networking service.
- Hardware refresh. Replacing an old router can surface the problem: newer models may need an extra module, or a converter, to terminate a DS3.
Questions to ask vendors
- Which of our circuits are DS3s, and what is each one used for today?
- Is the DS3 channelized or clear-channel, and if channelized, what is on each T1?
- Is this circuit affected by a retirement or price change, and on what timeline?
- Is fiber Ethernet available at this address, and what is the realistic installation lead time?
- Can the replacement be installed and tested before the DS3 is disconnected?
- If voice runs on the DS3, will our phone numbers port to the new service, and how long will that take?
- What early termination charges apply if we disconnect before the contract ends?
How it differs from a T1 line
A T1 and a DS3 belong to the same North American digital hierarchy, and a DS3 is built from 28 T1-rate signals. The practical differences are scale and delivery. A T1 runs at 1.544 Mbps and was traditionally delivered over two pairs of copper wire. A DS3 runs at about 45 Mbps and is usually handed off on coaxial cable from carrier equipment fed by fiber. Both are legacy services, and replacing either for data is typically a move to Ethernet; the DS3 project is just larger, and more likely to involve several services sharing one circuit.